A workstation of the type described above is known. It is constructed as a sealing station at a thermoforming machine. The tool is formed by an upper and lower part, whereby the lower part is driven with a stroke-type movement via a bell crank lever mechanism and acts upon the workpiece, namely the two foil webs, in the working position which represents the closed position of the tool. In doing this it is important that the workpiece is worked upon under compressive force. In order to achieve such a reproducible compressive force, spring arrangements are known which consist of one or more springs. These spring arrangements are provided in the path of one of the two parts of the tool and serve the purpose of creating the compressive force not via the bell crank lever mechanism in direct frame-type support on the workpiece, but rather to allow the force of the spring arrangement to act on the workpiece in the closed position of the tool which might be the working position. For this purpose, the spring arrangement is somewhat compressed in the working position. The spring arrangement is provided in a fettered position. It is known to arrange the prestress and the spring deflection path so that they may both be adjusted. By prestress is meant the force with which the spring arrangement is prestressed in the open position of the tool. By spring deflection path is meant the path through which the spring assembly was compressed into the working position. It is understood that the compressive force, corresponding to the compression or rather, the deflection path of the spring arrangement, exerted by the spring arrangement on the workpiece is greater than the prestress set in the open position of the tool. The setting of the prestress and the spring deflection path is carried out according to the dimensional specifications of the manufacturer of the workstation. As the synthetic foils forming the workpiece have different properties according to the particular case, i.e. in particular, different thicknesses or tolerance ranges for the thicknesses, it is necessary, for correct functioning of the workstation to readjust or rather set the compressive force, for example so that in a sealing station, the lidding foil is so sealed onto the cup foil that long-term tightness is achieved. Accordingly, the setting of the prestress is altered or rather, adapted to a process which is dependent on the skill of the operator. Alterations to the prestress as a consequence of aging, breakage, setting of the spring arrangement, etc. can have a disadvantageous effect on production results and can often remain unnoticed over long production cycles. It can also happen that a lidding foil only becomes detached after several days or even weeks so that appropriate inspections directly after sealing do not detect this error which occurs as a result of a deviation to the compressive force. The number of rejects which can be produced in this way can be considerable.
A workstation at a packaging machine, the basic arrangement of which is comparable to the foregoing, is also known from DE-AS 26 56 219. The spring arrangement is formed by a single Bellville spring washer, neither the prestress nor the spring deflection path of which are adjustable. It is not possible to satisfy the required operating conditions in this manner. However, the workstation does have the advantage of an overload safety device essentially consisting of two gear wheels which can be turned against each other so that, in one position, the teeth support each other but, however, in the other position, in which the gear wheels are offset against each other by half of one pitch, a power support is avoided.